Acrylic coatings have superior appearance, weatherability, and chemical resistance. They are widely used in automotive clearcoats, industrial maintenance topcoats, general metal, wood furniture and flooring coatings. There are two general types of acrylic coatings: acrylic-urethane and acrylic-melamine. Acrylic-urethane coatings are based on acrylic polyols and isocyanates. Acrylic-melamine coatings are based on acrylic polyols and melamines. Acrylic-urethane coatings are usually packaged as two separate parts (2K). One part contains an acrylic polyol and the other an isocyanate. These two parts are mixed prior to application.
UV-curable coatings are usually one-part (1K) systems which contain a mixture of acrylic monomers and acrylated oligomers. Acrylated oligomers are commonly prepared by reacting a polyether polyol or polyester polyol with a diisocyanate to form an NCO prepolymer, and the later then reacts with a hydroxyalkyl acrylate. The existence of polyether or polyester often reduces the weatherability of the UV-curable coatings.
Despite their superior resistance to weathering, acrylic polyols are rarely used for making NCO prepolymers. Unlike polyether or polyester polyols, acrylic polyols are made by radical chain polymerization of hydroxyalkyl acrylates or methacrylates and alkyl acrylates or methacrylates. The hydroxyl groups are randomly pendant along the polymer chain, rather than located only at the chain ends. Also, acrylic polyols usually have more than two OH groups per chain. Thus, the reaction of acrylic polyols with diisocyanates often results in gel even under well-controlled conditions. Hence, acrylic polyol-based acrylated oligomers are not often found in UV-curable coatings.
Dual-cure coatings are known. See Radtech Report, November-December 2001 issue. These coatings are based on acrylated polyisocyanates and unsaturated polyester polyols. They can be first UV-cured to obtain tack-free surface so that the coated parts can be handled, buffed, or sanded. The coatings are then fully cured through urethane reaction. However, the known dual-cure coatings are still based on acrylated polyester oligomers, and thus they are difficult to make and are expensive.
New dual-cure coatings are needed. Ideally, the dual-cure coatings would not need acrylated polyester or polyether oligomers. Ideally, the dual-cure coatings would not need any acrylated oligomers.